What you suggest is a bad standard as it requires a waste of time to determine the supported version of Perl when that is of no consequence.
I'd like to think a decent Perl 5 programmer has some idea of the version of the language to which he or she programs. If you can only deploy to 5.008, write use 5.008; at the start of all of your programs and no one will be tempted to use defined-or or state or given/when, whereas if you want to use any of those features, you'd better specify the language version.
Let's stop pretending there aren't mildly incompatible versions of Perl 5 The Language. feature has taken away that fiction.